Pretty sure Outlook and other tools use system colors for various color schemes. Once a Theme is set the colors in that theme are reflected in the system colors. So if you use Luna you get the blue background and light background color that very light
blue.

FWIW, this is pretty much moot with Vista, since Vista doesn't really 'do' Themes. Every themes aware app I've used uses the same generic grey background that you see on a Win2000 machine (or XP without Themes).

I think ideally you'd just want to let people choose from one of a few themes...